Re: [Qemu-devel] [PATCH 3/3] Delayed IP packets

[Amit Shah <amit.shah@redhat.com>]

On (Wed) 03 Aug 2011 [13:24:22], Jan Kiszka wrote:
> From: Fabien Chouteau <chouteau@adacore.com>
> 
> In the current implementation, if Slirp tries to send an IP packet to a client
> with an unknown hardware address, the packet is simply dropped and an ARP
> request is sent (if_encap in slirp/slirp.c).
> 
> With this patch, Slirp will send the ARP request, re-queue the packet and try
> to send it later. The packet is dropped after one second if the ARP reply is
> not received.

This patch causes a segfault when guests wake up from hibernate.

Recipe:
1. Start guest with -net user -net nic,model=virtio
2. (guest) ping 10.0.2.2
3. (guest) echo "disk" > /sys/power/state
4. Re-start guest with same command line
5. Ping has stopped receiving replies.
6. Kill that ping process and start a new one.  qemu segfaults.

This needs the not-upstream-yet virtio S4 handling patches, found at
http://thread.gmane.org/gmane.linux.kernel/1197141

The backtrace is:

(gdb) bt
#0  0x00007ffff7e421f7 in slirp_insque (a=0x0, b=0x7ffff8f95d50) at
/home/amit/src/qemu/slirp/misc.c:27
#1  0x00007ffff7e40738 in if_start (slirp=0x7ffff8a9cdf0) at
/home/amit/src/qemu/slirp/if.c:194
#2  0x00007ffff7e44828 in slirp_select_poll (readfds=0x7fffffffd930,
writefds=0x7fffffffd9b0, xfds=0x7fffffffda30, select_error=0)
    at /home/amit/src/qemu/slirp/slirp.c:588
#3  0x00007ffff7e110f1 in main_loop_wait (nonblocking=<optimized out>)
at /home/amit/src/qemu/vl.c:1549
#4  0x00007ffff7d7dc47 in main_loop () at
/home/amit/src/qemu/vl.c:1579
#5  main (argc=<optimized out>, argv=<optimized out>, envp=<optimized
out>) at /home/amit/src/qemu/vl.c:3574


Reverting the patch keeps the ping going on after resume.  

Leaving the patch in context:

> 
> Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>  slirp/if.c    |   28 +++++++++++++++++++---
>  slirp/main.h  |    2 +-
>  slirp/mbuf.c  |    2 +
>  slirp/mbuf.h  |    2 +
>  slirp/slirp.c |   72 +++++++++++++++++++++++++++++++-------------------------
>  5 files changed, 69 insertions(+), 37 deletions(-)
> 
> diff --git a/slirp/if.c b/slirp/if.c
> index 0f04e13..2d79e45 100644
> --- a/slirp/if.c
> +++ b/slirp/if.c
> @@ -6,6 +6,7 @@
>   */
>  
>  #include <slirp.h>
> +#include "qemu-timer.h"
>  
>  #define ifs_init(ifm) ((ifm)->ifs_next = (ifm)->ifs_prev = (ifm))
>  
> @@ -105,6 +106,9 @@ if_output(struct socket *so, struct mbuf *ifm)
>  	ifs_init(ifm);
>  	insque(ifm, ifq);
>  
> +        /* Expiration date = Now + 1 second */
> +        ifm->expiration_date = qemu_get_clock_ns(rt_clock) + 1000000000ULL;
> +
>  diddit:
>  	slirp->if_queued++;
>  
> @@ -153,6 +157,9 @@ diddit:
>  void
>  if_start(Slirp *slirp)
>  {
> +    int requeued = 0;
> +    uint64_t now;
> +
>  	struct mbuf *ifm, *ifqt;
>  
>  	DEBUG_CALL("if_start");
> @@ -165,6 +172,8 @@ if_start(Slirp *slirp)
>          if (!slirp_can_output(slirp->opaque))
>              return;
>  
> +        now = qemu_get_clock_ns(rt_clock);
> +
>  	/*
>  	 * See which queue to get next packet from
>  	 * If there's something in the fastq, select it immediately
> @@ -199,11 +208,22 @@ if_start(Slirp *slirp)
>  		   ifm->ifq_so->so_nqueued = 0;
>  	}
>  
> -	/* Encapsulate the packet for sending */
> -        if_encap(slirp, (uint8_t *)ifm->m_data, ifm->m_len);
> -
> -        m_free(ifm);
> +        if (ifm->expiration_date < now) {
> +            /* Expired */
> +            m_free(ifm);
> +        } else {
> +            /* Encapsulate the packet for sending */
> +            if (if_encap(slirp, ifm)) {
> +                m_free(ifm);
> +            } else {
> +                /* re-queue */
> +                insque(ifm, ifqt);
> +                requeued++;
> +            }
> +        }
>  
>  	if (slirp->if_queued)
>  	   goto again;
> +
> +        slirp->if_queued = requeued;
>  }
> diff --git a/slirp/main.h b/slirp/main.h
> index 0dd8d81..028df4b 100644
> --- a/slirp/main.h
> +++ b/slirp/main.h
> @@ -42,5 +42,5 @@ extern int tcp_keepintvl;
>  #define PROTO_PPP 0x2
>  #endif
>  
> -void if_encap(Slirp *slirp, const uint8_t *ip_data, int ip_data_len);
> +int if_encap(Slirp *slirp, struct mbuf *ifm);
>  ssize_t slirp_send(struct socket *so, const void *buf, size_t len, int flags);
> diff --git a/slirp/mbuf.c b/slirp/mbuf.c
> index ce2eb84..c699c75 100644
> --- a/slirp/mbuf.c
> +++ b/slirp/mbuf.c
> @@ -70,6 +70,8 @@ m_get(Slirp *slirp)
>  	m->m_len = 0;
>          m->m_nextpkt = NULL;
>          m->m_prevpkt = NULL;
> +        m->arp_requested = false;
> +        m->expiration_date = (uint64_t)-1;
>  end_error:
>  	DEBUG_ARG("m = %lx", (long )m);
>  	return m;
> diff --git a/slirp/mbuf.h b/slirp/mbuf.h
> index b74544b..55170e5 100644
> --- a/slirp/mbuf.h
> +++ b/slirp/mbuf.h
> @@ -86,6 +86,8 @@ struct mbuf {
>  		char	m_dat_[1]; /* ANSI don't like 0 sized arrays */
>  		char	*m_ext_;
>  	} M_dat;
> +    bool     arp_requested;
> +    uint64_t expiration_date;
>  };
>  
>  #define m_next		m_hdr.mh_next
> diff --git a/slirp/slirp.c b/slirp/slirp.c
> index 4a9a4d5..a86cc6e 100644
> --- a/slirp/slirp.c
> +++ b/slirp/slirp.c
> @@ -692,55 +692,63 @@ void slirp_input(Slirp *slirp, const uint8_t *pkt, int pkt_len)
>      }
>  }
>  
> -/* output the IP packet to the ethernet device */
> -void if_encap(Slirp *slirp, const uint8_t *ip_data, int ip_data_len)
> +/* Output the IP packet to the ethernet device. Returns 0 if the packet must be
> + * re-queued.
> + */
> +int if_encap(Slirp *slirp, struct mbuf *ifm)
>  {
>      uint8_t buf[1600];
>      struct ethhdr *eh = (struct ethhdr *)buf;
>      uint8_t ethaddr[ETH_ALEN];
> -    const struct ip *iph = (const struct ip *)ip_data;
> +    const struct ip *iph = (const struct ip *)ifm->m_data;
>  
> -    if (ip_data_len + ETH_HLEN > sizeof(buf))
> -        return;
> +    if (ifm->m_len + ETH_HLEN > sizeof(buf)) {
> +        return 1;
> +    }
>  
>      if (!arp_table_search(slirp, iph->ip_dst.s_addr, ethaddr)) {
>          uint8_t arp_req[ETH_HLEN + sizeof(struct arphdr)];
>          struct ethhdr *reh = (struct ethhdr *)arp_req;
>          struct arphdr *rah = (struct arphdr *)(arp_req + ETH_HLEN);
>  
> -        /* If the client addr is not known, there is no point in
> -           sending the packet to it. Normally the sender should have
> -           done an ARP request to get its MAC address. Here we do it
> -           in place of sending the packet and we hope that the sender
> -           will retry sending its packet. */
> -        memset(reh->h_dest, 0xff, ETH_ALEN);
> -        memcpy(reh->h_source, special_ethaddr, ETH_ALEN - 4);
> -        memcpy(&reh->h_source[2], &slirp->vhost_addr, 4);
> -        reh->h_proto = htons(ETH_P_ARP);
> -        rah->ar_hrd = htons(1);
> -        rah->ar_pro = htons(ETH_P_IP);
> -        rah->ar_hln = ETH_ALEN;
> -        rah->ar_pln = 4;
> -        rah->ar_op = htons(ARPOP_REQUEST);
> -        /* source hw addr */
> -        memcpy(rah->ar_sha, special_ethaddr, ETH_ALEN - 4);
> -        memcpy(&rah->ar_sha[2], &slirp->vhost_addr, 4);
> -        /* source IP */
> -        rah->ar_sip = slirp->vhost_addr.s_addr;
> -        /* target hw addr (none) */
> -        memset(rah->ar_tha, 0, ETH_ALEN);
> -        /* target IP */
> -        rah->ar_tip = iph->ip_dst.s_addr;
> -        slirp->client_ipaddr = iph->ip_dst;
> -        slirp_output(slirp->opaque, arp_req, sizeof(arp_req));
> +        if (!ifm->arp_requested) {
> +            /* If the client addr is not known, send an ARP request */
> +            memset(reh->h_dest, 0xff, ETH_ALEN);
> +            memcpy(reh->h_source, special_ethaddr, ETH_ALEN - 4);
> +            memcpy(&reh->h_source[2], &slirp->vhost_addr, 4);
> +            reh->h_proto = htons(ETH_P_ARP);
> +            rah->ar_hrd = htons(1);
> +            rah->ar_pro = htons(ETH_P_IP);
> +            rah->ar_hln = ETH_ALEN;
> +            rah->ar_pln = 4;
> +            rah->ar_op = htons(ARPOP_REQUEST);
> +
> +            /* source hw addr */
> +            memcpy(rah->ar_sha, special_ethaddr, ETH_ALEN - 4);
> +            memcpy(&rah->ar_sha[2], &slirp->vhost_addr, 4);
> +
> +            /* source IP */
> +            rah->ar_sip = slirp->vhost_addr.s_addr;
> +
> +            /* target hw addr (none) */
> +            memset(rah->ar_tha, 0, ETH_ALEN);
> +
> +            /* target IP */
> +            rah->ar_tip = iph->ip_dst.s_addr;
> +            slirp->client_ipaddr = iph->ip_dst;
> +            slirp_output(slirp->opaque, arp_req, sizeof(arp_req));
> +            ifm->arp_requested = true;
> +        }
> +        return 0;
>      } else {
>          memcpy(eh->h_dest, ethaddr, ETH_ALEN);
>          memcpy(eh->h_source, special_ethaddr, ETH_ALEN - 4);
>          /* XXX: not correct */
>          memcpy(&eh->h_source[2], &slirp->vhost_addr, 4);
>          eh->h_proto = htons(ETH_P_IP);
> -        memcpy(buf + sizeof(struct ethhdr), ip_data, ip_data_len);
> -        slirp_output(slirp->opaque, buf, ip_data_len + ETH_HLEN);
> +        memcpy(buf + sizeof(struct ethhdr), ifm->m_data, ifm->m_len);
> +        slirp_output(slirp->opaque, buf, ifm->m_len + ETH_HLEN);
> +        return 1;
>      }
>  }
>  
> -- 
> 1.7.3.4
> 
> 

		Amit